1. Field of the Invention
The present invention relates to the field of free piston shock tubes/tunnels, and more particularly, to a diaphragm construction for a free piston shock tube/tunnel.
2. Description of the Prior Art
Free piston shock tube/tunnels have existed since the 1950's. During operation, such shock tube/tunnels are able to generate a shock wave of extremely high pressure and high temperature at a test site for a desired duration or test time. Free piston shock tube/tunnels are principally used to provide aerodynamic test conditions for rocket nose cones, space re-entry vehicles, hypersonic aircraft and the like.
In general, free piston shock tube/tunnels include an elongated, generally cylindrical compression tube containing a compression or driver gas such as helium. The compression tube is normally closed at one end by a diaphragm having a preselected rupture pressure. A compression piston is contained within the compression tube and is adapted for movement from a piston end of the tube toward the diaphragm end. Connected to the diaphragm end of the compression tube is an elongated shock tube having a test end remote from the diaphragm and being filled with a low pressure driven gas such as ambient air. When the piston is moved from the piston starting end of the compression tube toward the diaphragm end, the gas within the compression tube is compressed, thus generating pressure and causing the diaphragm to rupture. The rupturing of the diaphragm causes a volume of the compression gas to pass through the ruptured diaphragm and into the connected shock tube to form a shock wave. The shock wave compresses the driven gas during movement through the shock tube, thereby creating the desired test conditions at the test site. In the case of the shock tunnel, the compressed gas is further processed through a nozzle at the final test site.
An area of free piston shock tubes/tunnels which has received considerable attention has been the area at the diaphragm end of the compression tube. Various improvements and developments have been made relative to this area of the shock tube/tunnel such as the provision of a replaceable orifice insert as disclosed in U.S. Pat. No. 5,115,665 and the provision of a piston stop member as disclosed in U.S. Pat. No. 5,245,868. However, diaphragm construction per se has remained relatively unchanged, with few variations or improvements throughout the years.
Although existing diaphragm design has been reasonably acceptable, high cost continues to be a problem, since the diaphragm must be replaced after each test, as well as the speed with which conventional diaphragms open. Conventional diaphragms have a single, centrally located rupture area designed to rupture along predetermined score lines when exposed to a predetermined pressure in the compression tube. Due primarily to the size of the diaphragm opening which is necessary to achieve certain shock formation and flow characteristics, it has been necessary to construct the diaphragm from relatively thick material such as steel or other metals. Further, the larger the opening, the longer it take for the opening to become fully opened when a rupture of the diaphragm occurs.
Accordingly, there is a need in the art for an improved diaphragm designed for a free piston shock tube/tunnel which is cheaper and which opens at least as quick, and preferably quicker, than conventional single opening diaphragms when exposed to the rupture pressure.